(a) Technical Field
The present invention relates to a method for controlling output of a fuel cell to improve fuel efficiency of a fuel cell hybrid vehicle. More particularly, the present invention relates to a method for controlling output of a fuel cell in a fuel cell hybrid vehicle that preferably includes a fuel cell as a main power source and a storage means (preferably a supercapacitor or battery) as an auxiliary power source, which can improve the fuel efficiency and the efficiency of a fuel cell system by effectively controlling the output of the fuel cell according to the level of energy stored in the storage means.
(b) Background Art
In general, a fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Such a fuel cell can be suitably applied to the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as to the supply of electric power for industry, home, and vehicle.
At present, the most attractive fuel cell generally in use for a vehicle is a polymer electrolyte membrane fuel cell (PEMFC), also called a proton exchange membrane fuel cell, comprising: a membrane electrode assembly (MEA) including a polymer electrolyte membrane (PEM) for transporting hydrogen ions and an electrode catalyst layer, in which an electrochemical reaction takes place, suitably disposed on both sides of the PEM; a gas diffusion layer (GDL) for uniformly diffusing reactant gases and transmitting generated electricity; a gasket and a sealing member for maintaining airtightness of the reactant gases and coolant and providing an appropriate bonding pressure; and a bipolar plate for transferring the reactant gases and coolant.
In the fuel cell having the above-described configuration, hydrogen as a fuel and oxygen (air) as an oxidizing agent are preferably supplied to an anode and a cathode through flow fields of the bipolar plate, respectively. The hydrogen is suitably supplied to the anode (also called a “fuel electrode”, “hydrogen electrode”, and “oxidation electrode”) and the oxygen (air) is suitably supplied to the cathode (also called an “air electrode”, “oxygen electrode”, and “reduction electrode”). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by catalyst of the electrode catalyst layer preferably provided on both sides of the electrolyte membrane. Preferably, only the hydrogen ions are selectively transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane and, at the same time, the electrons are suitably transmitted to the anode through the GDL and the bipolar plate, which are conductors. At the anode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate meet the oxygen in the air preferably supplied to the anode by an air supplier and suitably cause a reaction that produces water. Due to the movement of hydrogen ions occurring at this time, the flow of electrons through an external conducting wire occurs, and thus a current is generated.
If the fuel cell is used as the only suitable power source of an electric vehicle, the fuel cell powers all loads of the vehicle, which results in performance deterioration during operation, where the efficiency of the fuel cell is low. Moreover, during high speed operation where a high voltage is suitably required, a sufficient voltage required by a drive motor is not supplied due to a rapid decrease in output voltage, and thus decreases acceleration performance. Furthermore, if a sudden load is applied to the vehicle, the output voltage of the fuel cell drops or suddenly drops and suitably sufficient power is not supplied to the drive motor, thus decreasing vehicle performance (for example, a sudden change in load imposes a heavy burden on the fuel cell since electricity is generated by an electrochemical reaction). In addition, since the fuel cell has unidirectional output characteristics, it is difficult to recover energy from the drive motor during braking of the vehicle, thus decreasing the efficiency of the vehicle system.
Accordingly, a fuel cell hybrid vehicle has been developed. Preferably, a fuel cell hybrid vehicle includes a large vehicle, such as, but not limited to, a bus, as well as a small vehicle, and is equipped with a suitable storage means such as a high voltage battery or a supercapacitor as an auxiliary power source for providing suitable power required for driving the motor in addition to the fuel cell as a main power source. At present, a fuel cell-storage means hybrid vehicle that does not employ a power converter has been studied, and the fuel cell-storage means hybrid vehicle has, for example, high fuel efficiency (high regenerative braking, high efficiency of supercapacitor, and without the use of the power converter), an increase in durability of the fuel cell, high reliability control, and the like.
In examples where the hybrid vehicle in which the fuel cell and the storage means are directly connected, the fuel cell continuously outputs power at suitably constant level during driving. Preferably, if electric power is suitably sufficient, the storage means is charged with surplus power, whereas, if the electric power is insufficient, the storage means supplies the insufficient power to drive the vehicle.
Preferably, a fuel cell-supercapacitor hybrid vehicle in which a supercapacitor is employed as the storage means preferably includes a fuel cell suitably used as a main power source, a supercapacitor suitably used as an auxiliary power source, a supercapacitor precharge unit suitably interposed between a main bus terminal, which is an output port of the fuel cell, and the supercapacitor, and a motor control unit (MCU) (preferably with an inverter), which is a power module for rotating a drive motor, preferably connected to output ports of the fuel cell and the supercapacitor, producing a 3-phase pulse width modulation (PWM) by receiving direct current therefrom, and controlling the motor drive and the regenerative braking. Preferably, the supercapacitor precharge unit is used only to charge the discharged supercapacitor during initial start-up.
Accordingly, the above-described fuel cell-supercapacitor hybrid vehicle preferably uses the fuel cell as the main power source, which receives hydrogen from a hydrogen tank and air from an air blower to generate electricity by an electrochemical reaction between hydrogen and oxygen in the air. According to preferred embodiments of the invention, the drive motor and the MCU are directly connected to the fuel cell through the main bus terminal, and the supercapacitor is connected to the fuel cell through the supercapacitor precharge unit to provide power assist and regenerative braking.
The configuration of an exemplary fuel cell system will be briefly described herein. FIG. 1 shows an exemplary air supplier and a hydrogen supplier. As shown in the figure, dry air suitably supplied through an air blower 28 is humidified by a humidifier 29 and supplied to a cathode of a fuel cell stack 2. Preferably, exhaust gas of the cathode, humidified with water generated from the cathode, is delivered to the humidifier 29 and used to humidify dry air to be supplied to the cathode by the air blower 28.
Preferably, the hydrogen supplier comprises two lines. The first line supplies hydrogen to an anode of the fuel cell stack 2 through a low pressure regulator (LPR) 23, and a portion of hydrogen at an outlet port of the anode is recirculated through a recirculation blower 24. The second line supplies hydrogen at high pressure directly to the anode through a valve 25 and an ejector 26, and a portion of hydrogen at the outlet port of the anode is recirculated and supplied through the ejector 26.
Moreover, hydrogen remaining in the anode directly passes through an electrolyte membrane without generation of electricity and reacts with oxygen in the cathode, which is called “crossover”. In order to reduce the amount of hydrogen crossover, it is necessary to reduce the anode pressure during low power operation and increase the anode pressure during high power operation wherein the output of the fuel cell stack is increased. To this end, the low pressure regulator 23 is used singly when low pressure is required, and hydrogen at high pressure is supplied by controlling the valve 25 when high power is required or during hydrogen purging. The higher the anode (hydrogen) pressure is, the more the amount of hydrogen crossover is increased. Since the hydrogen crossover has undesirable effects on the fuel efficiency and durability of the fuel cell, it is necessary to maintain an appropriate anode pressure. Accordingly, a hydrogen purge valve 27 is preferably used to discharge impurities and condensed water at the anode, thus ensuring the performance of the fuel cell stack. The outlet port of the anode is suitably connected to a water trap 31 such that the condensed water stored in the water trap 31 is discharged through a valve 32 if the amount of condensed water reaches a suitable predetermined level.
Preferably, the driving mode of the hybrid vehicle including the fuel cell as the main power source and the supercapacitor (or a high voltage battery which is a secondary battery) as the auxiliary power source includes an electric vehicle (EV) mode in which the motor is driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is driven by the fuel cell and the supercapacitor at the same time, and a regenerative braking (RB) mode in which the supercapacitor is charged.
However, in the fuel cell-supercapacitor hybrid vehicle the supercapacitor is automatically charged by the fuel cell, which restricts the regenerative braking. Accordingly, such a problem can be solved by stopping the operation of the fuel cell during low power operation and during regenerative braking. Moreover, it is possible to improve the fuel efficiency by restricting the use of the fuel cell during low power operation where the efficiency of the fuel cell is low.
As above, in order to improve the fuel efficiency, it is necessary to consider the fuel cell stop/restart process, i.e., an idle stop/start control process, in which the power generation of the fuel cell is suitably stopped and restarted (the fuel cell is turned on and off), if necessary, during driving of the fuel cell-battery or fuel cell-supercapacitor hybrid vehicle. The idle stop of the fuel cell during driving of the vehicle is clearly distinguished from the shut-down of the fuel cell system after the vehicle operation is finished. Accordingly, it is necessary to distinguish a control process for the idle stop of the fuel cell from a control process for the shut-down of the fuel cell system.
In order to improve the fuel efficiency of the hybrid vehicle including the fuel cell and the storage means, U.S. Patent Publication No. 20030118876 is directed to a method in which a relay switch, connected between a fuel cell and a supercapacitor, is turned off to disconnect the output of the fuel cell during low power operation or if the voltage of the supercapacitor is above a predetermined level, and the relay switch is turned on to connect the output of the fuel cell if an output required by the vehicle is increased or if the voltage of the supercapacitor is below a predetermined level. In this technique, the relay switch of a main bus terminal for disconnecting the output of the fuel cell is turned on and off to achieve the idle stop/start, and thus a separate relay on/off control is required. The above technique does not perform any control other than connecting and disconnecting the output of the fuel cell by turning on and off the relay switch. Furthermore, the relay switch is turned off during regenerative braking and is turned on, if the voltage is above a predetermined level, in a state where the power generation of the fuel cell is made. Accordingly, a portion of the amount of regenerative braking is consumed in fuel cell balance-of-plant (BOP) components to be used in the power generation of the fuel cell, which is to prevent the voltage of the main bus terminal from rising.
U.S. Pat. No. 6,484,075 is directed to a technique in which the fuel cell power supply is cut off by determining an idle state based on a wheel rotational speed, whether or not a brake is operated, a state of charge (SOC), an electrical load, and the like, and the fuel cell power supply is restarted if a power storage unit is below a predetermined SOC. Here, the conditions for entering the idle stop are considerably restrictive (e.g., the idle stop is performed if the vehicle stopped, if the load is below a predetermined value, if the brake is in an operation state, and if the SOC is above a predetermined value). Moreover, a separate device such as a DC/DC chopper is required at the fuel cell for the idle stop, and the DC/DC chopper is directly connected to the supercapacitor during releasing the idle stop state after the DC/DC chopper is used to limit the current. The DC/DC chopper is a buck converter, which is restrictively used to limit the current when it is directly connected to the supercapacitor after cutting off the fuel cell current.
It is necessary to provide a method for suitably improving the fuel efficiency of the fuel cell system by effectively controlling the fuel cell by a more simplified control technique, for example as distinguished from conventional techniques.
Accordingly, to the present invention preferably provides a method for maximizing energy recovery during regenerative braking, wherein the method comprises a more simplified control technique, and is a method for effectively stopping the operation of the fuel cell therefor.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.